Effects of grouping in contextual modulation (Jont Allen )

http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v415/n6870/abs/415433a_r.html&dynoptions=
Effects of grouping in contextual modulation
MICHAEL H. HERZOG* AND MANFRED FAHLE*?
* Human Neurobiology, University of Bremen, Argonnenstrasse 3, 28211 Bremen, Germany
? City University, Department of Optometry and Visual Science, Tait Building, Northampton
Square, London EC1V 0HB, UK
Correspondence and requests for materials should be addressed to M.H.H. (e-mail:
mherzog(at)uni-bremen.de).
Perception of a visual target and the responses of cortical neurons can be strongly
influenced by a context
surrounding the target. This observation relates to the fundamental issue of how cortical
neurons code objects
of the external world. In high-contrast regimes, embedding a target in an iso-oriented
context reduces neural
responses and deteriorates performance in psychophysical experiments. Performance from
orthogonal
surrounds is better than that from iso-oriented ones. This contextual interference is
often postulated to be
caused by long- or short-range interactions between neurons tuned to orientation. Here we
show, using a
new illusion called 'shine-through' as a sensitive psychophysical probe, that the
orientation difference
between target and context does not determine performance. Instead, contextual modulation
depends on the
overall spatial structure of the context. We propose that contextual suppression vanishes
if the contextual
elements are grouped to an independent and coherent object.